Polarization scramblers are used in many applications relating to fiber-optic test and measurement. These scramblers typically utilize piezoelectric transducers (sometimes known as xe2x80x9csqueezersxe2x80x9d) and provide increasing benefit with increasing speed and/or efficiency in completely generating a plurality of states of polarization (SOP) that cover a substantial portion of the Poincare sphere of polarization states. Polarization scramblers in the prior art use inefficient, high-voltage techniques to drive the piezoelectric transducers. Such techniques have several disadvantages; first and foremost, the high voltage itself is a drawback. Second, when driving a capacitive load (i.e., the piezoelectric transducers) with high voltage, the scrambler""s speed of operation decreases due to bandwidth limitations.
The invention circumvents the afore-mentioned problems, in one object, by driving the piezoelectric transducers at resonant frequencies, resulting in higher-speed operation at lower voltages, as compared to the prior art. Other objects of the invention are apparent within the description that follows.
The following patents provide useful background information for the invention: U.S. Pat. Nos. 5,682,445; 5,633,959; 5,408,545; 5,159,481; 4,988,169; 4,979,235; 4,960,319; 4,923,290; 4,753,507; 4,753,507; 3,645,603; and 3,625,589. Each of the afore-mentioned patents is expressly incorporated herein by reference.
The following articles provide useful background information for the invention: M. Johnson, In-line fiber-optical polarization transformer, Appl. Opt. 18, p.1288 (1979); R. Ulrich, Polarization stabilization on single-mode fiber, Appl. Phys. Lett. 35, p. 840 (1979); Kidoh et al., Polarization control on ouptut of single-mode optical fibers, IEEE J. Quan. Elec. 17, p. 991 (1981); R. Alferness, Electrooptic guided-wave device for general polarization transformations, IEEE J. Quan. Elec. 17, p.965 (1981); Sakai et al., Birefringence and polarization characteristics of single-mode optical fibers under elastic deformations, IEEE J. Quan. Elec. 17, p.1041 (1981); R. Noe, Endless polarization control in coherent optical communications, Elec. Lett. 22, p.772 (1986); R. Noe, Endless polarization control experiment with three elements of limited birefringence range, Elec. Lett. 22, p.1341 (1986); N. Walker et al., Endless polarization control using four fiber squeezers, Elec. Lett. 23, p. 290 (1987); A. Kersey et al., Monomode fiber polarization scrambler, Elec. Lett. 23, p.634 (1987); Tatam et al., Full polarization state control utilizing linearly birefringent monomode optical fiber, IEEE J. Lightwave Tech. 5, p.980 (1987); G Walker et al., Rugged, all-fiber, endless polarization controller, Elec. Lett. 24, p.1353 (1988); 2xc3x972 Optical Fiber Polarization Switch and Polarization controller, Elec. Lett. 24, p.1427 (1988); and S. Siddiqui, Liquid crystal polarization controller for use in fiber communication systems, Optical Fiber Conference Proceedings, Wed. afternoon, poster #122 (1989). Each of the afore-mentioned articles is incorporated herein by reference.
This invention of one aspect provides a fiber-based polarization scrambler. The scrambler uses multiple piezoelectric squeezers that exert radial forces on a section of single-mode optical fiber. The radial forces on the fiber change the fiber""s birefringence via the photoelastic effect, which changes the SOP of light transmitted through the section of squeezed fiber. In the preferred aspect of the invention, each of the piezoelectric squeezers is excited independently at one of its resonant frequencies by an electronic drive. Each squeezer may be driven at the same or different frequencies from every other squeezer. Use of resonant frequencies in the polarization scrambler of the invention thus reduces the drive voltages required to change the SOP, and yet provides for higher speed and efficiency, as compared to prior art polarization scramblers.
In another aspect of the invention, the electronic drive controls amplitude output independently from drive frequency, for each squeezer. In a related aspect, the electronic drive controls amplitude output signals substantially independently from drive frequency, for each squeezer.
In still another aspect, the polarization scrambler of the invention creates a plurality of polarization statesxe2x80x94sometimes referred to as coverage of the Poincare sphere. Preferably, the plurality of polarization states includes all polarization states to cover substantially all of the Poincare sphere. One measure of the effectiveness of the scrambler is the Degree of Polarization (DOP), defined in Born et al., Principles of Optics, 6th Edition, Pergamon Press, p.554-555 (1980):       D    ⁢          xe2x80x83        ⁢    O    ⁢          xe2x80x83        ⁢    P    =                                          ⟨                          S              1                        ⟩                    2                +                              ⟨                          S              2                        ⟩                    2                +                              ⟨                          S              3                        ⟩                    2                            S      0      
where the Si are components of the Stokes vector that describes the SOP at a given moment in time, and the  less than  greater than  indicate the average of the Stokes component over a measurement time-interval. Minimizing DOP over a measurement time interval (1/measurement bandwidth) depends on generating a plurality of polarization states within the time they are measured. In one aspect of the invention, the plurality of polarization states are produced in a time less than about 100 milliseconds, and preferably less than 1 millisecond. The DOP will be less than about 5% at a measurement bandwidth of 10 kHz. In a further aspect, the scrambler generates a pattern or sequence of polarization states; these patterns or sequences may be random or periodic in time.
The invention of one aspect provides an improvement to a polarization sensitive optical measurement system. Such a system can include, for example, polarization-dependent loss devices or polarization-mode-dispersion measuring devices. In accord with the invention, the optical measurement system incorporates a polarization scrambler, such as described above, to quickly and efficiently induce coverage across the Poincare sphere, detuning or eliminating system sensitivity to polarization effects. Those skilled in the art should appreciate that the improvement provided by the polarization scrambler of the invention can provide further enhancements to other fiber optical systems and instruments of the prior art.
In yet another aspect, the invention provides an improvement to an optical source. Such a source can include, for example, a laser diode, LED, or amplified spontaneous emission devices.
A key consideration for users of polarization scramblers within the fiber optic marketplace relates to speed of operation; since the invention provides improved operating speed over prior art polarization scramblers, the invention provides obvious advantages.
The invention is next described further in connection with preferred embodiments, and it will become apparent that various additions, subtractions, and modifications can be made by those skilled in the art without departing from the scope of the invention.